Electrophotography is a kind of image-forming process, which generally comprises charging the surface of a photoreceptor containing a photoconductive material in the dark by means of, e.g., corona discharge, image-wise exposing the resulting photoreceptor to selectively eliminate the charge in the exposed area to thereby obtain an electrostatic latent image, Converting the latent image into a visible image using a toner, transferring the toner image to paper, etc., and then fixing the toner particles to obtain an image.
Photoreceptors include inorganic photoreceptors containing an inorganic photoconductive compound, e.g., selenium, zinc oxide, cadmium sulfide, or silicon, as a major component and organic photoreceptors containing an organic charge generation material and a low- or high-molecular organic charge transporting material both of which are dispersed in a binder resin. The inorganic photoreceptors each has many advantages and has been widely used so far. However, those inorganic photoconductive compounds have the following drawbacks. For example, selenium not only is costly because of the difficulty of the production thereof, but also tends to crystallize and to be readily affected by heat or mechanical shock to thereby suffer performance deterioration. Zinc oxide and cadmium sulfide are insufficient in moisture resistance and mechanical strength, and a dye added as a sensitizer is deteriorated by the charging and exposure. Thus, photoreceptors containing zinc oxide or cadmium sulfide are defective in durability, etc. Silicon is also costly because of the difficulty of the production thereof and because a highly irritant gas is used for producing the same. Moreover, care should be taken in handling silicon because it is sensitive to moisture.
For the purpose of overcoming the drawbacks of these inorganic photoreceptors, organic photoreceptors containing various organic compounds have been investigated in recent years and have come to be used widely. The organic photoreceptors include single-layer photoreceptors in which both a charge generation material and a charge transporting material are dispersed in a binder resin and double-layered photoreceptors which comprise a charge generation layer and a charge transporting layer which layers perform their respective functions. Organic photoreceptors of the double-layer type are advantageous in that each material can be selected from a wide range of compounds and a photoreceptor having a desired performance can be produced relatively easily by selecting a suitable material combination. Because of this, a large number of investigations have been made on double-layered organic photoreceptors, which are in wide use.
As the charge generation materials, various kinds of organic pigments and dyes have been proposed and put to practical use. Examples thereof include azo compounds, bisazo compounds, trisazo compounds, tetrakisazo compounds, thiapyrylium salts, squarilium salts, azulenium salts, cyanine dyes, perylene compounds, metal-free or metal phthalocyanine compounds, polynuclear quinone compounds, thioindigo compounds, and quinacridone compounds.
Examples of charge transporting materials include the oxadiazole compounds disclosed in JP-B-34-5466, the oxazole compounds disclosed in JP-A-56-123544, pyrazoline compounds disclosed in JP-B-52-41880, the hydrazone compounds disclosed in JP-B-55-42380, JP-B-61-40104, JP-B-62-35673, and JP-B-63-35976, the diamine compounds disclosed in JP-B-58-32372, stilbene compounds disclosed in JP-B-63-18738, JP-B-63-19867, and JP-B-3-39306, and the butadiene compounds disclosed in JP-A-62-30255. (The terms "JP-B" and "JP-A" as used herein mean an "examined Japanese patent publication" and an "unexamined published Japanese patent application," respectively.) Some of the organic photoreceptors containing these charge transporting materials have excellent properties and have come into practical use. However, any organic photoreceptor has not been obtained so far which fully satisfies the various property requirements which an electrophotographic photoreceptor is required to meet.
A charge transporting material for use in an organic photoreceptor is required not only to enable the photoreceptor to satisfy various property requirements as a photoreceptor including sensitivity, but also to have chemical stability so as to withstand light, ozone, and electrical load and further have stability or durability so as not to cause a sensitivity decrease even when the photoreceptor is used repeatedly or over long.